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The article discusses the strong force, which is the strongest force in the universe and binds atomic nuclei together. Despite limited knowledge about its precise strength, recent breakthroughs have allowed scientists to measure and understand the strong force more accurately. The force changes with distance and is responsible for most of the visible mass in the universe. By studying the interaction between quarks and gluons, researchers have made predictions and calculations that were previously impossible. This new understanding of the strong force may also have implications for explaining phenomena outside of our current understanding. [Extracted from the article]

We use the twist dimensions of the operators underlying the dynamical behavior of exclusive production processes as a tool for determining the structure of exotic heavy-quark states such as the Z+c(4430) tetraquark. The resulting counting rules predict distinctive falloffs of the cross sections in center-of-mass energy, thus distinguishing whether the tetraquarks are segregated into di-meson molecules, diquark-antidiquark pairs, or more democratically arranged four-quark states. In addition, we propose straightforward methods of experimentally producing additional exotic multiquark states. [ABSTRACT FROM AUTHOR]

I discuss a number of novel tests of QCD at the LHC, measurements which can illuminate fundamental features of hadron physics. I also review the “Principle of Maximum Conformality” (PMC) which systematically sets the renormalization scale order-by-order in pQCD, eliminating an unnecessary theoretical uncertainty. The PMC allows LHC experiments to test QCD much more precisely, and the sensitivity of LHC measurements to physics beyond the Standard Model is increased.

Abstract: Light-Front Hamiltonian theory, derived from the quantization of the QCD Lagrangian at fixed light-front time , provides a rigorous frame-independent framework for solving nonperturbative QCD. The eigenvalues of the light-front QCD Hamiltonian predict the hadronic mass spectrum, and the corresponding eigensolutions provide the light-front wavefunctions which describe hadron structure. The valence Fock-state wavefunctions of the light-front QCD Hamiltonian satisfy a single-variable relativistic equation of motion, analogous to the nonrelativistic radial Schrödinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. In fact, the potential U has a unique form if one requires that the action for zero quark mass remains conformally invariant. We also show that the holographic mapping of gravity in AdS space to QCD with a specific soft-wall dilaton yields the same light-front Schrödinger equation. Light-front holography also leads to a precise relation between the bound-state amplitudes in the fifth dimension z of AdS space and the boost-invariant light-front wavefunctions describing the internal structure of hadrons in physical space-time. The elastic and transition form factors of the pion and the nucleons are found to be well described in this framework. The predictions of the LF equations of motion include a zero-mass pion in the chiral limit, and linear Regge trajectories with the same slope in the radial quantum number n and orbital angular momentum L. The light-front AdS/QCD holographic approach thus gives a frame-independent representation of color-confining dynamics, Regge spectroscopy, and the excitation spectra of relativistic light-quark meson and baryon bound states in QCD in terms of a single mass parameter. We also briefly discuss the implications of the underlying conformal template of QCD for renormalization scale-setting and the implications of light-front quantization for the value of the cosmological constant. [Copyright &y& Elsevier]

We present in detail a new systematic method which can be used to automatically eliminate the renormalization scheme and scale ambiguities in perturbative QCD predictions at all orders. We show that all of the nonconformal β-dependent terms in a QCD perturbative series can be readily identified by generalizing the conventional renormalization schemes based on dimensional regularization. We then demonstrate that the nonconformal series of pQCD at any order can be resummed systematically into the scale of the QCD coupling in a unique and unambiguous way due to a special degeneracy of the β terms in the series. The resummation follows from the principal of maximum conformality (PMC) and assigns a unique scale for the running coupling at each perturbative order. The final result is independent of the initial choices of renormalization scheme and scale, in accordance with the principles of the renormalization group, and thus eliminates an unnecessary source of systematic error in physical predictions. We exhibit several examples known to order as4 i.e. (i) the electron-positron annihilation into hadrons, (ii) the tau-lepton decay to hadrons, (iii) the Bjorken and Gross-Llewellyn Smith (GLS) sum rules, and (iv) the static quark potential. We show that the final series of the first three cases are all given in terms of the anomalous dimension of the photon field in SU(N), in accordance with conformality, and with all nonconformal properties encoded in the running coupling. The final expressions for the Bjorken and GLS sum rules directly lead to the generalized Crewther relations, exposing another relevant feature of conformality. The static quark potential shows that PMC scale-setting in the Abelian limit is to all orders consistent with QED scale-setting. Finally, we demonstrate that the method applies to any renormalization scheme and can be used to derive commensurate scale relations between measurable effective charges, which provide nontrivial tests of QCD to high precision. This work extends Brodsky-Lepage-Mackenzie (BLM) scale-setting to any perturbative order, with no ambiguities in identifying β terms in pQCD, demonstrating that BLM scale-setting follows from a principle of maximum conformality. [ABSTRACT FROM AUTHOR]

We examine in detail the diagrammatic mechanisms which provide, the change of sign between the single transverse spin asymmetries measured in semi-inclusive deep inelastic scattering (SIDIS) and in the Drell-Yan process (DY). This asymmetry is known to arise due to the transverse spin dependence of the target proton combined with a T-odd complex phase. XJsing the discrete symmetry properties of transverse spinors, we show that the required complex phase originates in the denominators of rescattering diagrams and their respective cuts. For simplicity, we work in a model where the proton consists of a valence quark and a scalar diquark. We then show that the phases generated in SIDIS and in DY originate from distinctly different cuts in the amplitudes, which at first appears to obscure the relationship between the single-spin asymmetries in the two processes. Nevertheless, further analysis demonstrates that the contributions of these cuts are identical in the leading-twist Bjorken kinematics considered, resulting in the standard sign-flip relation between the Sivers functions in SIDIS and DY. Physically, this fundamental, but yet untested, prediction occurs because the Sivers effect in the Drell-Yan reaction is modified by the initial-state "lensing" interactions of the annihilating antiquark, in contrast to the final-state lensing which produces the Sivers effect in deep inelastic scattering. [ABSTRACT FROM AUTHOR]

A key problem in making precise perturbative QCD predictions is the uncertainty in determining the renormalization scale μ of the running coupling αs(μ²). The purpose of the running coupling in any gauge theory is to sum all terms involving the β function; in fact, when the renormalization scale is set properly, all nonconformal β ≠ 0 terms in a perturbative expansion arising from renormalization are summed into the running coupling. The remaining terms in the perturbative series are then identical to that of a conformai theory; i.e., the corresponding theory with β = 0. The resulting scale-fixed predictions using the principle of maximum conformality (PMC) are independent of the choice of renormalization scheme--a key requirement of renormalization group invariance. The results avoid renormalon resum-mation and agree with QED scale setting in the Abelian limit. The PMC is also the theoretical principle underlying the Brodsky-Lepage-Mackenzie procedure, commensurate scale relations between observ-ables, and the scale-setting method used in lattice gauge theory. The number of active flavors nf in the QCD β function is also correctly determined. We discuss several methods for determining the PMC scale for QCD processes. We show that a single global PMC scale, valid at leading order, can be derived from basic properties of the perturbative QCD cross section. The elimination of the renormalization scale ambiguity and the scheme dependence using the PMC will not only increase the precision of QCD tests, but it will also increase the sensitivity of collider experiments to new physics beyond the Standard Model. [ABSTRACT FROM AUTHOR]

In conventional treatments, predictions from fixed-order perturbative QCD calculations cannot be fixed with certainty because of ambiguities in the choice of the renormalization scale as well as the renormalization scheme. In this paper we present a general discussion of the constraints of the renormalization group (RG) invariance on the choice of the renormalization scale. We adopt the RG-based equations, which incorporate the scheme parameters, for a general exposition of RG invariance, since they simultaneously express the invariance of physical observables under both the variation of the renormalization scale and the renormalization scheme parameters. We then discuss the self-consistency requirements of the RG, such as reflexivity, symmetry, and transitivity, which must be satisfied by the scale-setting method. The principle of minimal sensitivity requires the slope of the approximant of an observable to vanish at the renormalization point. This criterion provides a scheme-independent estimation, but it violates the symmetry and transitivity properties of the RG and does not reproduce the Gell-Mann-Low scale for QED observables. The principle of maximum conformality (PMC) satisfies all of the deductions of the RG invariance--reflectivity, symmetry, and transitivity. Using the PMC, all non-conformal {βRi} terms CR stands for an arbitrary renormalization scheme) in the perturbative expansion series are summed into the running coupling, and one obtains a unique, scale-fixed, scheme-independent prediction at any finite order. The PMC scales and the resulting finite-order PMC predictions are both to high accuracy independent of the choice of initial renormalization scale, consistent with RG- invariance. Moreover, after PMC scale setting, the residual initial scale dependence at fixed order owing to unknown higher-order {βRi} terms can be substantially suppressed. The PMC thus eliminates a serious systematic scale error in perturbative quantum chromodynamics predictions, greatly improving the precision of tests of the Standard Model and the sensitivity to new physics at collider and other experiments. [ABSTRACT FROM AUTHOR]

A major contribution to the uncertainty of finite-order perturbative QCD predictions is the perceived ambiguity in setting the renormalization scale &mgr;. For example, by using the conventional way of setting &mgr;r ∈ [mt/2, 2mt], one obtains the total tt production cross section &sgr;t&tmacr;, with the uncertainty &Dgr;&sgr;t&tmacr;,/&sgr;t&tmacr; ∼ (…) at the Tevatron and LHC even for the present next-to next-to-leading-order level. The principle of maximum conformality (PMC) eliminates the renormalization scale ambiguity in precision tests of Abelian QED and non-Abelian QCD theories. By using the PMC, all nonconformal {&bgr;i} terms in the perturbative expansion series are summed into the running coupling constant, and the resulting scale-fixed predictions are independent of the renormalization scheme. The correct scale displacement between the arguments of different renormalization schemes is automatically set, and the number of active flavors nf in the {&bgr;i} function is correctly determined. The PMC is consistent with the renormalization group property that a physical result is independent of the renormalization scheme and the choice of the initial renormalization scale &mgr;init The PMC scale &mgr;PMCr is unambiguous at finite order. Any residual dependence on &MGR;INIT for a finite-order calculation will be highly suppressed since the unknown higher-order {&bgr;i} terms will be absorbed into the PMC scales' higher-order perturbative terms. We find that such renormalization group invariance can be satisfied to high accuracy for &sgr;t&tmacr; at the next-to next-to-leading-order level. In this paper we apply PMC scale setting to predict the tt cross section &sgr;t&tmacr; at the Tevatron and LHC colliders. It is found that a,f remains almost unchanged by varying &mgr;init within the region of [mt/4,4mt]. The convergence of the expansion series is greatly improved. For the (qq) channel, which is dominant at the Tevatron, its next-to-leading-order (NLO) PMC scale is much smaller than the top-quark mass in the small x region, and thus its NLO cross section is increased by about a factor of 2. In the case of the (gg) channel, which is dominant at the LHC, its NLO PMC scale slightly increases with the subprocess collision energy √s, but it is still smaller than mt, for √s ≲ 1 TeV, and the resulting NLO cross section is increased by ∼20%. As a result, a larger &sgr;t&tmacr; is obtained in comparison to the conventional scale setting method, which agrees well with the present Tevatron and LHC data. More explicitly, by setting mt = 172.9 ± 1.1 GeV, we predict &sgr;Tevatrorl 196TeV = 7.626+0.265-0.257 pb, &sgr;LHC,7TeV = 171.8+5.8-5.6 pb and &sgr;LHC,14TeV = 941.3287-2.65 Pb. [ABSTRACT FROM AUTHOR]

The renormalization scale uncertainty can be eliminated by the principle of maximum conformality (PMC) in a systematic, scheme-independent way. Applying the PMC for the t&tmacr;-pair hadroproduction at the next-to-next-to-leading-order level, we have found that the total cross sections &sgr;t&tmacr; at both the Tevatron and LHC remain almost unchanged when taking very disparate initial scales &mgr;initR equal to mt 10mt, 20mt, and √s, which is consistent with renormalization group invariance. As an important new application, we apply PMC scale setting to study the top quark forward-backward asymmetry. We observe that the more convergent perturbative series after PMC scale-setting leads to a more accurate top quark forwardbackward asymmetry. The resulting PMC prediction on the asymmetry is also free from the initial renormalization scale dependence. Because the next-to-leading-order PMC scale has a dip behavior for the (q&qmacr;) channel at small subprocess collision energies, the importance of this channel to the asymmetry is increased. We observe that the asymmetries At&tmacr;FB and Ap&pmacr;FB at the Tevatron will be increased by 42% in comparison to the previous estimates obtained by using conventional scale setting; i.e., we obtain At&tmacr;PMCFB ≃ 12.5% and Ap&pmacr;PMCFB ≃ 8.28%. Moreover, we obtain At&tmacr;PMCFB(Mt&tmacr;>450GeV) ≃ 35.0%. These predictions have a 1 cr deviation from the present CDF and D0 measurements; the large discrepancies of the top quark forward-backward asymmetry between the standard model estimate and the CDF and D0 data are thus greatly reduced. [ABSTRACT FROM AUTHOR]

A key problem in making precise perturbative QCD predictions is to set the proper renormalization scale of the running coupling. The extended renormalization group equations, which express the invariance of the physical observables under both the renormalization scale- and scheme-parameter transformations, provide a convenient way for estimating the scale- and scheme-dependence of the physical process. In this paper, we present a solution for the scale equation of the extended renormalization group equations at the four-loop level. Using the principle of maximum conformality (PMC)/ Brodsky-Lepage-Mackenzie (BLM) scale-setting method, all nonconformal {/3,} terms in the perturbative expansion series can be summed into the running coupling, and the resulting scale-fixed predictions are independent of the renormalization scheme. The PMC/BLM scales can be fixed order-by-order. As a useful reference, we present a systematic and scheme-independent procedure for setting PMC/BLM scales up to next-to-next-to-leading order. An explicit application for determining the scale setting of Ree+e- (Q) up to four loops is presented. By using the world average &agr;&MSmacr;s(Mz) = 0.1184 ± 0.0007, we obtain the asymptotic scale for the 't Hooft scheme associated with the &MSmacr; scheme, &Dgr;&MSmacr;uH/MS = 245+19-10 MeV, and the asymptotic scale for the conventional MS&MSmacr; scheme, &Dgr;sub>&MSmacr; = 213+19-8 MeV. [ABSTRACT FROM AUTHOR]

We study the photon-to-meson transition form factors (TFFs) FMγ(Q²) for γγ* → M using light-front holographic methods. The Chern-Simons action, which is a natural form in five-dimensional anti--de Sitter (ADS) space, is required to describe the anomalous coupling of mesons to photons using holographic methods and leads directly to an expression for the photon-to-pion TFF for a class of confining models. Remarkably, the predicted pion TFF is identical to the leading order QCD result where the distribution amplitude has asymptotic form. The Chern-Simons form is local in AdS space and is thus somewhat limited in its predictability. It only retains the qq̄ component of the pion wave function, and further, it projects out only the asymptotic form of the meson distribution amplitude. It is found that in order to describe simultaneously the decay process π0 → γγ and the pion TFF at the asymptotic limit, a probability for the qq̄ component of the pion wave function Pqq̄ = 0.5 is required, thus giving indication that the contributions from higher Fock states in the pion light-front wave function need to be included in the analysis. The probability for the Fock state containing four quarks Pqq̄qq̄ ~ 10%, which follows from analyzing the hadron matrix elements for a dressed current model, agrees with the analysis of the pion elastic form factor using light-front holography including higher Fock components in the pion wave function. The results for the TFFs for the η and η' mesons are also presented. The rapid growth of the pion TFF exhibited by the BABAR data at high Q² is not compatible with the models discussed in this article, whereas the theoretical calculations are in agreement with the experimental data for the η and η' TFFs. [ABSTRACT FROM AUTHOR]

The QCD evolution of the pion distribution amplitude (DA) φπ(x, Q²) is computed for several commonly used models. Our analysis includes the nonperturbative form predicted by light-front holographic QCD, thus combining the nonperturbative bound state dynamics of the pion with the perturbative ERBL evolution of the pion DA. We calculate the meson-photon transition form factors for the π0, η and η' using the hard-scattering formalism. We point out that a widely-used approximation of replacing φ(x, (1 - x)Q) with φ(x, Q) in the calculations will unjustifiably reduce the predictions for the meson-photon transition form factors. It is found that the four models of the pion DA discussed give very different predictions for the Q² dependence of the meson-photon transition form factors in the region of Q² > 30 GeV². More accurate measurements of these transition form factors at the large Q² region will be able to distinguish different models of the pion DA. The rapid growth of the large Q² data for the pion-photon transition form factor reported by the BABAR Collaboration is difficult to explain within the current framework of QCD. If the BABAR data for the pion-photon transition form factor is confirmed, it could indicate physics beyond-the-standard model, such as a weakly coupled elementary C = + axial vector or pseudoscalar z0 in the few GeV domain, an elementary field which would provide the coupling γ*γ → z0 → π0 at leading twist. Our analysis thus indicates the importance of additional measurements of the pion-photon transition form factor at large Q². [ABSTRACT FROM AUTHOR]

We show how expansions in powers of Planck's constant ħ = h/2π can give new insights into perturbative and nonperturbative properties of quantum field theories. Since ħ is a fundamental parameter, exact Lorentz invariance and gauge invariance are maintained at each order of the expansion. The physics of the ħ expansion depends on the scheme; i.e., different expansions are obtained depending on which quantities (momenta, couplings, and masses) are assumed to be independent of ħ. We show that if the coupling and mass parameters appearing in the Lagrangian density are taken to be independent of ħ, then each loop in perturbation theory brings a factor of ħ. In the case of quantum electrodynamics, this scheme implies that the classical charge e, as well as the fine structure constant are linear in ħ. The connection between the number of loops and factors of ħ is more subtle for bound states since the binding energies and bound-state momenta themselves scale with ħ. The ħ expansion allows one to identify equal-time relativistic bound states in QED and QCD which are of lowest order in ħ and transform dynamically under Lorentz boosts. The possibility to use retarded propagators at the Born level gives valencelike wave functions which implicitly describe the sea constituents of the bound states normally present in its Fock state representation. [ABSTRACT FROM AUTHOR]

The scaling behavior of the invariant production cross section is a powerful tool in order to probe the dynamics of particle production. Here, we investigate the scaling properties of large-p ⊥ hadron, jet and prompt photon production in hadronic collisions by comparing systematically world data to NLO QCD predictions. In the hadron sector a significant discrepancy is reported, while prompt photon and jet production data prove in agreement with leadingtwist expectations. We interpret these results as coming from a non-negligible contribution of higher-twist processes, in which the hadron is produced directly in the hard subprocess. Predictions at RHIC are successfully compared to PHENIX preliminary measurements and LHC predictions are given. [ABSTRACT FROM AUTHOR]

Starting from the bound state Hamiltonian equation of motion in QCD, we derive relativistic light-front wave equations in terms of an invariant impact variable ζ which measures the separation of the quark and gluonic constituents within the hadron at equal light-front time. These equations of motion in physical space-time are equivalent to the equations of motion which describe the propagation of spin-J modes in anti–de Sitter (AdS) space. Its eigenvalues give the hadronic spectrum, and its eigenmodes represent the probability amplitudes of the hadronic constituents at a given scale. An effective classical gravity description in a positive-sign dilaton background is given for the phenomenologically successful soft-wall model which naturally encodes the internal structure of hadrons and their orbital angular momentum. Applications to the light meson and baryon spectrum are presented. [Copyright &y& Elsevier]

AdS/QCD, the correspondence between theories in a modified five-dimensional anti-de Sitter space and confining field theories in physical space-time, provides a remarkable semiclassical model for hadron physics. Light-front holography allows hadronic amplitudes in the AdS fifth dimension to be mapped to frame-independent light-front wavefunctions of hadrons in physical space-time, thus providing a relativistic description of hadrons at the amplitude level. We identify the AdS coordinate z with an invariant light-front coordinate ζ which separates the dynamics of quark and gluon binding from the kinematics of constituent spin and internal orbital angular momentum. The result is a single-variable light-front Schrödinger equation with a confining potential which determines the eigenspectrum and the light-front wavefunctions of hadrons for general spin and orbital angular momentum. The mapping of electromagnetic and gravitational form factors in AdS space to their corresponding expressions in light-front theory confirms this correspondence. Some novel features of QCD are discussed, including the consequences of confinement for quark and gluon condensates. The distinction between static structure functions, such as the probability distributions computed from the square of the light-front wavefunctions, versus dynamical structure functions which include the effects of rescattering, is emphasized. A new method for computing the hadronization of quark and gluon jets at the amplitude level, an event amplitude generator, is outlined. [Copyright &y& Elsevier]

Light-front Fock state wavefunctions encode the bound state properties of hadrons in terms of their quark and gluon degrees of freedom at the amplitude level. The freedom to choose the light-like quantization four-vector provides an explicitly covariant formulation of light-front quantization and can be used to determine the analytic structure of light-front wave functions. The AdS/CFT correspondence of large NC supergravity theory in higher-dimensional anti-de Sitter space with supersymmetric QCD in 4-dimensional space-time has interesting implications for hadron phenomenology in the conformal limit, including an all-orders demonstration of counting rules for exclusive processes. String/gauge duality also predicts the QCD power-law behavior of light-front Fock-state hadronic wavefunctions with arbitrary orbital angular momentum at high momentum transfer. The form of these near-conformal wavefunctions can be used as an initial ansatz for a variational treatment of the light-front QCD Hamiltonian. I also briefly review recent work which shows that some leading-twist phenomena such as the diffractive component of deep inelastic scattering, single spin asymmetries, nuclear shadowing and antishadowing cannot be computed from the LFWFs of hadrons in isolation. [Copyright &y& Elsevier]

Heisenberg's matrix formulation of quantum mechanics can be generalized to relativistic systems by evolving in light-front time τ = t + z/c. The spectrum and wavefunctions of bound states, such as hadrons in quantum chromodynamics, can be obtained from matrix diagonalization of the light-front Hamiltonian on a finite dimensional light-front Fock basis defined using periodic boundary conditions in x— and x⊥. This method, discretized light-cone quantization (DLCQ), preserves the frame-independence of the front form even at finite resolution and particle number. Light-front quantization can also be used in the Hamiltonian form to construct an event generator for high energy physics reactions at the amplitude level. The light-front partition function, summed over exponentially-weighted light-front energies, has simple boost properties which may be useful for studies in heavy ion collisions. I also review recent work which shows that the structure functions measured in deep inelastic lepton scattering are affected by final-state rescattering, thus modifying their connection to light-front probability distributions. In particular, the shadowing of nuclear structure functions is due to destructive interference effects from leading-twist diffraction of the virtual photon, physics not included in the nuclear light-front wavefunctions.

The breaking of chiral symmetry in holographic light-front QCD is encoded in its longitudinal dynamics, with its chiral limit protected by the superconformal algebraic structure which governs its transverse dynamics. The scale in the longitudinal light-front Hamiltonian determines the confinement strength in this direction: It is also responsible for most of the light meson ground state mass consistent with the Gell-Mann-Oakes-Renner constraint. Longitudinal confinement and the breaking of chiral symmetry are found to be different manifestations of the same underlying dynamics as found in the 't Hooft large-NC QCD (1+1) model. [ABSTRACT FROM AUTHOR]

A new algorithm for the automatic computation of Feynman diagram amplitudes in quantum field theory is presented. Once the topology of a diagram is defined, the algorithm constructs all corresponding light-cone time-orderings. We explore the method for two- and three-loop calculations in QED. Copyright 1993, 1999 Academic Press

We identify a property of renormalizable SU(N)/U(1) gauge theories, intrinsic conformality (iCF), which underlies the scale invariance of physical observables and leads to a remarkably efficient method to solve the conventional renormalization scale ambiguity at every order in perturbative QCD (pQCD): the PMC∞. This new method reflects the underlying conformal properties displayed by pQCD at next-to-next-to-leading order, eliminates the scheme dependence of pQCD predictions, and is consistent with the general properties of the principle of maximum conformality (PMC). We introduce a new method to identify conformal and β-terms which can be applied from either a numerical or an analytical calculations. We illustrate the PMC∞ for the thrust and C-parameter distributions in e+e- annihilation and then show how to apply this new method to general observables in QCD. We point out how the implementation of the PMC∞ can significantly improve the precision of pQCD predictions; its implementation in a multiloop analysis also simplifies the calculation of higher order corrections in a general renormalizable gauge theory. [ABSTRACT FROM AUTHOR]

Heavy fermion pair production in e+e- annihilation is a fundamental process in hadron physics and is of considerable interest for various phenomena. In this paper, we will apply the principle of maximum conformality (PMC) to provide a comprehensive analysis of these processes. The PMC provides a systematic, unambiguous method for determining the renormalization scales of the QCD coupling constant for single-scale and multiple-scale applications. The resulting predictions eliminate any renormalization scheme-and-scale ambiguities, eliminate the factorial renormalon divergences, and are consistent with the requirements of the renormalization group. It is remarkable that two distinctly different scales are determined by using the PMC for heavy fermion pair production near the threshold region. One scale is the order of the fermion mass mf, which enters the hard virtual corrections, and the other scale is of order vmf, where v is the quark velocity, which enters the Coulomb rescattering amplitude. The PMC scales yield the correct physical behavior and reflect the virtuality of the propagating gluons (photons) for the QCD (QED) processes. Moreover, we demonstrate the consistency of PMC scale setting from QCD to QED. Perfect agreement between the Abelian unambiguous Gell-Mann-Low and the PMC scale-setting methods in the limit of zero number of colors is demonstrated. [ABSTRACT FROM AUTHOR]

We present a novel method for precisely determining the running QCD coupling constant αs(Q²) over a wide range of Q² from event shapes for electron-positron annihilation measured at a single annihilation energy √s. The renormalization scale Q² of the running coupling depends dynamically on the virtuality of the underlying quark and gluon subprocess and thus the specific kinematics of each event. The determination of the renormalization scale for event shape distributions is obtained by using the principle of maximum conformality (PMC), a rigorous scale-setting method for gauge theories which satisfies all the requirements of renormalization group invariance, including renormalization-scheme independence and consistency with Abelian theory in the NC→0 limit. In this paper, we apply the PMC to two classic event shapes measured in e+e- annihilation: the thrust (T) and C-parameter (C). The PMC renormalization scale depends differentially on the values of T and C. The application of PMC scale-setting determines the running coupling αs(Q²) to high precision over a wide range of Q² from 10 to 250 GeV² from measurements of the event shape distributions at the Z0 peak. The extrapolation of the running coupling using pQCD evolution gives the value αs(MZ²)=0.1185±0.0012 from the thrust and αs(MZ²)=0.1193-0.0019+0.0021 from the C-parameter in the MS̅ scheme. These determinations of αs(MZ²) are consistent with the world average and are more precise than the values obtained from analyses of event shapes currently used in the world average. The highly consistent results for the T and C event-shape distributions provide an additional verification of the applicability of the PMC to pQCD. [ABSTRACT FROM AUTHOR]

We present a comprehensive and self-consistent analysis for the thrust distribution by using the principle of maximum conformality (PMC). By absorbing all nonconformal terms into the running coupling using PMC via renormalization group equation, the scale in the running coupling shows the correct physical behavior and the correct number of active flavors is determined. The resulting PMC predictions agree with the precise measurements for both the thrust differential distributions and the thrust mean values. Moreover, we provide a new remarkable way to determine the running of the coupling constant αs(Q²) from the measurement of the jet distributions in electron-positron annihilation at a single given value of the center-of-mass energy √s. [ABSTRACT FROM AUTHOR]

Relativistic light-front bound-state equations for double-heavy mesons, baryons and tetraquarks are constructed in the framework of supersymmetric light front holographic QCD. Although heavy quark masses strongly break conformal symmetry, supersymmetry and the holographic embedding of semiclassical light-front dynamics still holds. The theory, derived from five-dimensional anti-de Sitter space, predicts that the form of the confining potential in the light-front Hamiltonian is harmonic even for heavy quarks. Therefore, the basic underlying supersymmetric mechanism, which transforms meson-baryon and baryon-tetraquark wave functions into each other, can also be applied to the double-heavy sector; one can then successfully relate the masses of the double-heavy mesons to double-heavy baryons and tetraquarks. The dependence of the confining potential on the hadron mass scale agrees completely with the one derived in heavy light systems from heavy quark symmetry. We also make predictions for higher excitations of the charmonium and bottomonium states. In particular, the remarkable equality of the Regge slopes in the orbital angular momentum, L, and the principal quantum number, n, is predicted to remain valid. [ABSTRACT FROM AUTHOR]

One of the remarkable features of high-multiplicity hadronic events in proton-proton collisions at the LHC is the fact that the produced particles appear as two "ridges", opposite in azimuthal angle ϕ, with approximately flat rapidity distributions. This phenomenon can be identified with the inelastic collision of gluonic flux tubes associated with the QCD interactions responsible for quark confinement in hadrons. In this paper, we analyze the ridge phenomena when the collision involves a flux tube connecting the quark and antiquark of a high energy real or virtual photon. We discuss gluonic tube string collisions in the context of two examples: electron-proton scattering at a future electron-ion collider or the peripheral scattering of protons accessible at the LHC. A striking prediction of our analysis is that the azimuthal angle of the produced ridges will be correlated with the scattering plane of the electron or proton producing the virtual photon. In the case of ep→eX, the final state X is expected to exhibit maximal multiplicity when the elliptic flow in X is aligned with the electron scattering plane. In the pp→ppX example, the multiplicity and elliptic flow in X are estimated to exhibit correlated oscillations as functions of the azimuthal angle Φ between the proton scattering planes. In the minimum-bias event samples, the amplitude of oscillations is expected to be on the order of 2% to 4% of the mean values. In the events with highest multiplicity, the oscillations can be three times larger than in the minimum-bias event samples. [ABSTRACT FROM AUTHOR]

Through the embedding of superconformal quantum mechanics into AdS space, it is possible to construct an effective supersymmetric QCD light-front Hamiltonian for hadrons, which includes a spin-spin interaction between the hadronic constituents. A specific breaking of conformal symmetry determines a unique effective quark-confining potential for light hadrons, as well as remarkable connections between the meson, baryon, and tetraquark spectra. The pion is massless in the chiral limit and has no supersymmetric partner. The excitation spectra of relativistic light-quark meson, baryon and tetraquark bound states lie on linear Regge trajectories with identical slopes in the radial and orbital quantum numbers. Although conformal symmetry is strongly broken by the heavy quark mass, the basic underlying supersymmetric mechanism, which transforms mesons to baryons (and baryons to tetraquarks) into each other, still holds and gives remarkable connections across the entire spectrum of light, heavy-light and double-heavy hadrons. Here we show that all the observed hadrons can be related through this effective supersymmetric QCD, and that it can be used to identify the structure of the new charmonium states. [ABSTRACT FROM AUTHOR]

QCD constituent counting rules define the scaling behavior of exclusive hadronic scattering and electromagnetic scattering amplitudes at high momentum transfer in terms of the total number of fundamental constituents in the initial and final states participating in the hard subprocess. The scaling laws reflect the twist of the leading Fock state for each hadron and hence the leading operator that creates the composite state from the vacuum. Thus, the constituent counting scaling laws can be used to identify the twist of exotic hadronic candidates such as tetraquarks and pentaquarks. Effective field theories must consistently implement the scaling rules in order to be consistent with the fundamental theory. Here, we examine how one can apply constituent counting rules for the exclusive production of one or two neutral vector mesons V0 in e+e- annihilation, processes in which the V0 can couple via intermediate photons. In the case of a (narrow) real V0, the photon virtuality is fixed to a precise value s1=m²V0, thus treating the V0 as a single fundamental particle. Each real V0 thus contributes to the constituent counting rules with NV0=1. In effect, the leading operator underlying the V0 has twist 1. Thus, in the specific physical case of single or double on-shell V0 production via intermediate photons, the predicted scaling from counting rules coincides with vector-meson dominance (VMD), an effective theory that treats V0 as an elementary field. However, the VMD prediction fails in the general case where the V0 is not coupled through an elementary photon field, and then the leading-twist interpolating operator has twist NV0=2. Analogous effects appear in pp scattering processes. [ABSTRACT FROM AUTHOR]

The discovery of extraterrestrial neutrinos in the ∼30 TeV-PeV energy range by IceCube provides new constraints on high energy astrophysics. An important background to the signal are the prompt neutrinos which originate from the decay of charm hadrons produced by high energy cosmic-ray particles interacting in the Earth's atmosphere. It is conventional to use the calculations of charm hadroproduction using gluon splitting g→cc alone. However, QCD predicts an additional "intrinsic" component of the heavy quark distribution which arises from diagrams where heavy quarks are multiply connected to the proton's valence quarks. We estimate the prompt neutrino spectrum due to intrinsic charm. We find that the atmospheric prompt neutrino flux from intrinsic charm is comparable to those calculated using QCD computations not including intrinsic charm, once we normalize the intrinsic charm differential cross sections to the ISR and the LEBC-MPS collaboration data. In the future, IceCube will constrain the intrinsic charm content of the proton and will contribute to one of the major questions in high energy physics phenomenology. [ABSTRACT FROM AUTHOR]

In this paper the dynamical spin effects of the light-front holographic wave functions for light pseudoscalar mesons are studied. These improved wave functions are then confronted with a number of hadronic observables: the decay constants of π and K mesons, their ξ-moments, the pion-to-photon transition form factor, and the pure annihilation Bs → π+π- and Bd → K+K- decays. Taking fπ, fK, and their ratio fK/fπ as constraints, we perform a χ² analysis for the holographic parameters, including the mass scale parameter √λ and the effective quark masses, and find that the fitted results are quite consistent with the ones obtained from the light-quark hadronic Regge trajectories. In addition, we also show that the end point divergence appearing in the pure annihilation Bs → π+π- and Bd → K+K- decays can be controlled well by using these improved light-front holographic distribution amplitudes. [ABSTRACT FROM AUTHOR]

We prove the Lorentz invariance of the angular momentum conservation law and the helicity sum rule for relativistic composite systems in the light-front formulation. We explicitly show that j ³, the z-component of the angular momentum remains unchanged under Lorentz transformations generated by the light-front kinematical boost operators. The invariance of j ³ under Lorentz transformations is a feature unique to the front form. Applying the Lorentz invariance of the angular quantum number in the front form, we obtain a selection rule for the orbital angular momentum which can be used to eliminate certain interaction vertices in QED and QCD. We also generalize the selection rule to any renormalizable theory and show that there exists an upper bound on the change of orbital angular momentum in scattering processes at any fixed order in perturbation theory. [ABSTRACT FROM AUTHOR]

Conformal symmetry provides a systematic approximation to QCD in both its perturbative and nonperturbative domains. One can use the AdS/CFT correspondence between Anti-de Sitter space and conformal gauge theories to obtain an analytically tractable approximation to QCD in the regime where the QCD coupling is large and constant. For example, there is an exact correspondence between the fifth-dimensional coordinate of AdS space and a specific impact variable which measures the separation of the quark constituents within the hadron in ordinary space-time. This connection allows one to compute the analytic form of the frame-independent light-front wave functions of mesons and baryons, the fundamental entities which encode hadron properties and allow the computation of exclusive scattering amplitudes. One can also use conformal symmetry as a template for perturbative QCD predictions where the effects of the nonzero beta function can be systematically included in the scale of the QCD coupling. This leads to fixing of the renormalization scale and commensurate scale relations which relate observables without scale or scheme ambiguity. The results are consistent with the renormalization group and the analytic connection of QCD to Abelian theory at NC →0. I also discuss a number of novel phenomenological features of QCD. Initial- and final-state interactions from gluon-exchange, normally neglected in the parton model, have a profound effect in QCD hard-scattering reactions, leading to leading-twist single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam Tung relation in Drell-Yan reactions, and nuclear shadowing and non-universal antishadowing – leading-twist physics not incorporated in the light-front wave functions of the target computed in isolation. I also discuss tests of hidden color in nuclear wave functions, the use of diffraction to materialize the Fock states of a hadronic projectile and test QCD color transparency, nonperturbative antisymmetric sea quark distributions, anomalous heavy quark effects, and the unexpected effects of direct higher-twist processes.

Relativistic light-front bound-state equations for mesons and baryons can be constructed in the chiral limit from the supercharges of a superconformal algebra which connect baryon and meson spectra. Quark masses break the conformal invariance, but the basic underlying supersymmetric mechanism, which transforms meson and baryon wave functions into each other, still holds and gives remarkable connections across the entire spectrum of light and heavy-light hadrons. We also briefly examine the consequences of extending the supersymmetric relations to double-heavy mesons and baryons. [ABSTRACT FROM AUTHOR]

We construct an effective QCD light-front Hamiltonian for both mesons and baryons in the chiral limit based on the generalized supercharges of a superconformal graded algebra. The superconformal construction is shown to be equivalent to a semi-classical approximation to light-front QCD and its embedding in AdS space. The specific breaking of conformal invariance inside the graded algebra uniquely determines the effective confinement potential. The generalized supercharges connect the baryon and meson spectra to each other in a remarkable manner. In particular, the π/b1 Regge trajectory is identified as the superpartner of the nucleon trajectory. However, the lowest-lying state on this trajectory, the p-meson is massless in the chiral limit and has no supersymmetric partner. [ABSTRACT FROM AUTHOR]

We describe the observed light-baryon spectrum by extending superconformal quantum mechanics to the light front and its embedding in AdS space. This procedure uniquely determines the confinement potential for arbitrary half-integer spin. To this end, we show that fermionic wave equations in AdS space are dual to light-front supersymmetric quantum-mechanical bound-state equations in physical space-time. The specific breaking of conformal invariance explains hadronic properties common to light mesons and baryons, such as the observed mass pattern in the radial and orbital excitations, from the spectrum generating algebra. The holographic embedding in AdS also explains distinctive and systematic features, such as the spin-J degeneracy for states with the same orbital angular momentum, observed in the light-baryon spectrum. [ABSTRACT FROM AUTHOR]

We observe that the linear potential used as a leading approximation for describing color confinement in the instant form of dynamics corresponds to a quadratic confining potential in the front form of dynamics. In particular, the instant-form potentials obtained from lattice gauge theory and string models of hadrons agree with the potentials determined from models using front-form dynamics and light-front holography, not only in their shape, but also in their numerical strength. [ABSTRACT FROM AUTHOR]

A complete calculation of the O(α4s) perturbative quantum chromodynamics (pQCD) corrections to the hadronic decay width of the Z boson has recently been performed by Baikov et al. [Phys. Rev. Lett. 108, 222003 (2012)]. In their analysis, Baikov et al. relied on the conventional practice of simply guessing the renormalization scale and taking an arbitrary range to estimate the pQCD uncertainties. This procedure inevitably introduces an arbitrary, scheme-dependent theoretical systematic error in the predictions. In this paper, we show that the renormalization scale uncertainties for hadronic Z decays can be greatly reduced by applying the principle of maximum conformality (PMC), a rigorous extension of the Brodsky-Lepage-Mackenzie method. The PMC prediction is independent of the choice of renormalization scheme; i.e., it respects renormalization group invariance, and thus it provides an optimal and theoretically rigorous method for setting the renormalization scale. We show that the convergence of the pQCD prediction for the Z hadronic width is greatly improved using the PMC since the divergent renormalon series does not appear. The magnitude of the high-order corrections quickly approach a steady point. The PMC predictions also have the property that any residual dependence on the choice of initial scale is highly suppressed, even for low-order predictions. Thus, one obtains optimal fixed-order predictions for the Z-boson hadronic decay rates thus enabling high precision tests of the Standard Model. [ABSTRACT FROM AUTHOR]

In this paper we derive holographic wave equations for hadrons with arbitrary spin starting from an effective action in a higher-dimensional space asymptotic to anti-de Sitter (AdS) space. Our procedure takes advantage of the local tangent frame, and it applies to all spins, including half-integer spins. An essential element is the mapping of the higher-dimensional equations of motion to the light-front Hamiltonian, thus allowing a clear distinction between the kinematical and dynamical aspects of the holographic approach to hadron physics. Accordingly, the nontrivial geometry of pure AdS space encodes the kinematics, and the additional deformations of AdS space encode the dynamics, including confinement. It thus becomes possible to identify the features of holographic QCD, which are independent of the specific mechanisms of conformal symmetry breaking. In particular, we account for some aspects of the striking similarities and differences observed in the systematics of the meson and baryon spectra. [ABSTRACT FROM AUTHOR]

We extend our analysis of the implications of hadronic supersymmetry for heavy-light hadrons in light-front holographic QCD. Although conformal symmetry is strongly broken by the heavy quark mass, supersymmetry and the holographic embedding of semiclassical light-front dynamics derived from five-dimensional anti-de Sitter space nevertheless determine the form of the confining potential in the light-front Hamiltonian to be harmonic. The resulting light-front bound-state equations lead to a heavy-light Regge-like spectrum for both mesons and baryons. The confinement hadron mass scale and their Regge slopes depend, however, on the mass of the heavy quark in the meson or baryon as expected from heavy quark effective theory. This procedure reproduces the observed spectra of heavy-light hadrons with good precision and makes predictions for yet unobserved states. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of the spacelike nucleon electromagnetic form factors and their flavor decomposition within the framework of light-front (LF) holographic QCD (LFHQCD) We show that the inclusion of the higher Fock components |qqqq?q has a significant effect on the spin-flip elastic Pauli form factor and almost zero effect on the spin-conserving Dirac form factor. We present light-front holographic QCD results for the proton and neutron form factors at any momentum transfer range, including asymptotic predictions, and show that our results agree with the available experimental data with high accuracy. In order to correctly describe the Pauli form factor we need an admixture of a five quark state of about 30% in the proton and about 40% in the neutron. We also extract the nucleon charge and magnetic radii and perform a flavor decomposition of the nucleon electromagnetic form factors. The free parameters needed to describe the experimental nucleon form factors are very few: two parameters for the probabilities of higher Fock states for the spin-flip form factor and a phenomenological parameter r, required to account for possible SU(6) spin-flavor symmetry breaking effects in the neutron, whereas the Pauli form factors are normalized to the experimental values of the anomalous magnetic moments. The covariant spin structure for the Dirac and Pauli nucleon form factors prescribed by AdS5 semiclassical gravity incorporates the correct twist scaling behavior from hard scattering and also leads to vector dominance at low energy. [ABSTRACT FROM AUTHOR]

We present improved perturbative QCD (pQCD) predictions for Higgs boson hadroproduction at the LHC by applying the principle of maximum conformality (PMC), a procedure which resums the pQCD series using the renormalization group (RG), thereby eliminating the dependence of the predictions on the choice of the renormalization scheme while minimizing sensitivity to the initial choice of the renormalization scale. In previous pQCD predictions for Higgs boson hadroproduction, it has been conventional to assume that the renormalization scale μr of the QCD coupling αs(μr) is the Higgs mass and then to vary this choice over the range 1/2mH<μr<2mH in order to estimate the theory uncertainty. However, this error estimate is only sensitive to the nonconformal β terms in the pQCD series, and thus it fails to correctly estimate the theory uncertainty in cases where a pQCD series has large higher-order contributions, as is the case for Higgs boson hadroproduction. Furthermore, this ad hoc choice of scale and range gives pQCD predictions which depend on the renormalization scheme being used, in contradiction to basic RG principles. In contrast, after applying the PMC, we obtain next-to-next-to-leading-order RG resummed pQCD predictions for Higgs boson hadroproduction which are renormalization-scheme independent and have minimal sensitivity to the choice of the initial renormalization scale. Taking mH=125 GeV, the PMC predictions for the pp→HX Higgs inclusive hadroproduction cross sections for various LHC center-of-mass energies are σIncl|7 TeV=21.21+1.36â?'1.32 pb, σIncl|8 TeV=27.37+1.65â?'1.59 pb, and σIncl|13 TeV=65.72+3.46â?'3.01 pb. We also predict the fiducial cross section σfid(pp→H→γγ): σfid|7 TeV=30.1+2.3â?'2.2 fb, σfid|8 TeV=38.3+2.9â?'2.8 fb, and σfid|13 TeV=85.8+5.7â?'5.3 fb. The error limits in these predictions include the small residual high-order renormalization-scale dependence plus the uncertainty from the factorization scale. The PMC predictions show better agreement with the ATLAS measurements than the LHC Higgs Cross Section Working Group predictions which are based on conventional renormalization-scale setting. [ABSTRACT FROM AUTHOR]

A key problem in making precise perturbative QCD (pQCD) predictions is how to set the renormalization scale of the running coupling unambiguously at each finite order. The elimination of the uncertainty in setting the renormalization scale in pQCD will greatly increase the precision of collider tests of the Standard Model and the sensitivity to new phenomena. Renormalization group invariance requires that predictions for observables must also be independent on the choice of the renormalization scheme. The well-known Brodsky-Lepage-Mackenzie (BLM) approach cannot be easily extended beyond next-to-next-to-leading order of pQCD. Several suggestions have been proposed to extend the BLM approach to all orders. In this paper we discuss two distinct methods. One is based on the "Principle of Maximum Conformality" (PMC), which provides a systematic all-orders method to eliminate the scale and scheme ambiguities of pQCD. The PMC extends the BLM procedure to all orders using renormalization group methods; as an outcome, it significantly improves the pQCD convergence by eliminating renormalon divergences. An alternative method is the "sequential extended BLM" (seBLM) approach, which has been primarily designed to improve the convergence of pQCD series. The seBLM, as originally proposed, introduces auxiliary fields and follows the pattern of the β0-expansion to fix the renormalization scale. However, the seBLM requires a recomputation of pQCD amplitudes including the auxiliary fields; due to the limited availability of calculations using these auxiliary fields, the seBLM has only been applied to a few processes at low orders. In order to avoid the complications of adding extra fields, we propose a modified version of seBLM which allows us to apply this method to higher orders. We then perform detailed numerical comparisons of the two alternative scale-setting approaches by investigating their predictions for the annihilation cross section ratio Re+e- at four-loop order in pQCD. [ABSTRACT FROM AUTHOR]

The principle of maximum conformality (PMC) provides a systematic and process-independent method to derive renormalization scheme- and scale-independent fixed-order pQCD predictions. By Brodsky and Wu [Phys. Rev. D 85, 114040 (2012)], we studied the top-quark charge asymmetry at the Tevatron. By applying the PMC, we have shown that the large discrepancies for the top-quark charge asymmetry between the Standard Model estimate and the CDF and D0 data are greatly reduced. In the present paper, with the help of the Bernreuther-Si program, we present a detailed PMC analysis on the top-quark pair production up to next-to-next-to-leading-order level at the LHC. After applying PMC scale setting, the pQCD prediction for the top-quark charge asymmetry at the LHC has very small scale uncertainty; e.g., AC|7 TeV;PMC = (1.15â'0.03+0.01)%, AC|8 TeV;PMC = (1.03+0.00+0.01)%, and AC|14 TeV;PMC = (0.62â'0.02+0.00)%. The corresponding predictions using conventional scale setting are: AC|7 TeV;Conv = (1.23â'0.14+0.14)%, AC|8 TeV;Conv = (1.11â'0.13+0.17)%, and AC|14 TeV;Conv = (0.67â'0.05+0.05)%. In these predictions, the scale errors are predicted by varying the initial renormalization and factorization scales in the ranges μrinit ∈ [mt/2,2mt] and μf ∈ [mt/2,2mt]. The PMC predictions are also in better agreement with the available ATLAS and CMS data. In addition, we have calculated the top-quark charge asymmetry assuming several typical cuts on the top-pair invariant mass Mtt. For example, assuming Mtt > 0.5 TeV and μf = μrinit = mt, we obtain AC|7 TeV;PMC = 2.67%, AC|8 TeV;PMC = 2.39%, and AC|14 TeV;PMC = 1.28%. [ABSTRACT FROM AUTHOR]

Abstract: Hamiltonian light-front quantum field theory provides a framework for calculating both static and dynamic properties of strongly interacting relativistic systems. Invariant masses, correlated parton amplitudes and time-dependent scattering amplitudes, possibly with strong external time-dependent fields, represent a few of the important applications. By choosing the light-front gauge and adopting an orthonormal basis function representation, we obtain a large, sparse, Hamiltonian matrix eigenvalue problem for mass eigenstates that we solve by adapting ab initio no-core methods of nuclear many-body theory. In the continuum limit, the infinite matrix limit, we recover full covariance. Guided by the symmetries of light-front quantized theory, we adopt a two-dimensional harmonic oscillator basis for transverse modes that corresponds with eigensolutions of the soft-wall anti-de Sitter/quantum chromodynamics (AdS/QCD) model obtained from light-front holography. We outline our approach and present results for non-linear Compton scattering, evaluated non-perturbatively, where a strong and time-dependent laser field accelerates the electron and produces states of higher invariant mass i.e. final states with photon emission. [Copyright &y& Elsevier]

The holographic light-front QCD framework provides a unified nonperturbative description of the hadron mass spectrum, form factors and quark distributions. In this article we extend holographic QCD in order to describe the gluonic distribution in both the proton and pion from the coupling of the metric fluctuations induced by the spin-two Pomeron with the energy momentum tensor in anti-de Sitter space, together with constraints imposed by the Veneziano model, without additional free parameters. The gluonic and quark distributions are shown to have significantly different effective QCD scales. [ABSTRACT FROM AUTHOR]

In this article a systematic quantitative analysis of the isoscalar bosonic states is performed in the framework of supersymmetric light front holographic QCD. It is shown that the spectroscopy of the η and h mesons can be well described if one additional mass parameter--which corresponds to the hard breaking of chiral U(1) symmetry in standard QCD--is introduced. The mass difference of the η and η′ isoscalar mesons is then determined by the strange quark mass content of the η′. The theory also predicts the existence of isoscalar tetraquarks which are bound states of diquarks and antidiquarks. The candidates for these exotic isoscalar tetraquarks are identified. In particular, the f0(1500) is identified as isoscalar tetraquark; the predicted mass value 1.52 GeV agrees with the measured experimental value within the model uncertainties. [ABSTRACT FROM AUTHOR]

We demonstrate that a nonzero strangeness contribution to the spacelike electromagnetic form factor of the nucleon is evidence for a strange-antistrange asymmetry in the nucleon's light-front wave function, thus implying different nonperturbative contributions to the strange and antistrange quark distribution functions. A recent lattice QCD calculation of the nucleon strange quark form factor predicts that the strange quark distribution is more centralized in coordinate space than the antistrange quark distribution, and thus the strange quark distribution is more spread out in light-front momentum space. We show that the lattice prediction implies that the difference between the strange and antistrange parton distribution functions, s(x)-¯s(x), is negative at small-x and positive at large-x. We also evaluate the strange quark form factor and s(x)-¯s(x) using a baryon-meson fluctuation model and a novel nonperturbative model based on light-front holographic QCD. This procedure leads to a Veneziano-like expression of the form factor, which depends exclusively on the twist of the hadron and the properties of the Regge trajectory of the vector meson which couples to the quark current in the hadron. The holographic structure of the model allows us to introduce unambiguously quark masses in the form factors and quark distributions preserving the hard scattering counting rule at large-Q² and the inclusive counting rule at large-x. Quark masses modify the Regge intercept which governs the small-x behavior of quark distributions, therefore modifying their small-x singular behavior. Both nonperturbative approaches provide descriptions of the strange-antistrange asymmetry and intrinsic strangeness in the nucleon consistent with the lattice QCD result. [ABSTRACT FROM AUTHOR]